Solid rocket motors include energetic and non-energetic materials. Improving the performance of a solid rocket motor typically requires increasing the performance of the energetic material, increasing the mass of energetic material, decreasing the mass of the non-energetic material, or some combination of these modifications. Because solid rocket motors are volume-limited systems, reducing the volume of non-energetic materials in the solid rocket motor allows for an increase in the volume and mass of energetic materials.
The non-energetic materials in a rocket motor may include, for example, a casing, insulation material, liner materials formulated to promote bonding, and nozzle materials. Reducing the volume of the insulation material may leave a relatively larger volume within the rocket motor, but may also leave the casing with insufficient thermal protection. Thus, in the design of rocket motors, performance and thermal protection are considered together in attempting to develop an optimized system within known parameters.
Rocket motor casings are generally made of metal, a composite material, or a combination of metal and composite materials. During operation, insulation protects the rocket motor casing from thermal effects and erosive effects of particle streams generated by combustion of a propellant. Typically, the insulation is bonded to the interior surface of the casing and is fabricated from a composition that, upon curing, is capable of enduring the high temperature gases and erosive particles produced while the propellant burns. A liner bonds the propellant to the insulation and to any noninsulated interior surface portions of the casing. Liners also typically have an ablative function, inhibiting burning of the insulation at liner-to-insulation interfaces.
The combustion of a solid rocket propellant generates extreme conditions within the rocket motor casing. For example, temperatures inside the rocket motor casing can reach 2,760° C. (5,000° F.). These conditions, in combination with the restrictive throat region of the nozzle passageway, create a high degree of turbulence of high-temperature combustion gases within the rocket motor casing and nozzle. In addition, gases produced during propellant combustion typically contain high-energy particles that, under a turbulent environment such as encountered in a rocket motor, can erode the rocket motor insulation. If gases produced by the burning propellant penetrate the insulation and liner, the casing may melt or otherwise be compromised, causing the rocket motor to fail. Thus, the insulation is formulated to withstand the extreme conditions experienced during propellant combustion and protect the casing from the burning propellant.
Some conventional rocket motor insulations include filled and unfilled plastics or polymers, such as phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, as well as ceramics, polyester resins, and the like. Plastics, however, tend to crack or blister in response to the rapid heat and pressure fluctuations experienced during rocket motor propellant combustion.
Rubbers and elastomers have also been used as rocket motor insulation. Cured ethylene-propylene-diene monomer (“EPDM”) terpolymer may be used alone or in a blend, and is often selected because of its favorable mechanical, thermal, and ablative properties. However, in high-velocity environments, the ablative properties of elastomers are sometimes inadequate for rocket motor operation unless the elastomers are reinforced with suitable fillers, such as carbon fibers or silica fibers. The criticality of avoiding high erosion rates is demonstrated by the severity and magnitude of risk of failure due to erosion. Most insulation is, of necessity, “man-rated” in the sense that a catastrophic failure can result in the loss of human life. Additionally, the tensile strength and tear strength of unfilled elastomers may not be sufficiently high to withstand and endure the mechanical stresses that the elastomer is subjected to during processing.
Incorporation of fibers can increase the ablation resistance of an insulation material. However, many fibers are friable, and degrade during the preparation of the insulation material.
It would be advantageous to provide a thermal protection system that occupies less volume than conventional insulation materials. Such thermal protection may make it possible to increase the volume loading of energetic material in a rocket motor, increasing the performance of the motor. Such thermal protection may also be useful to make advanced propellant formulations feasible (e.g., propellants that burn hotter than conventional propellants).